Section I Part B

Question 18 (4 marks)

This is an extended response question, so you need to plan the answer. There are no step by step instructions given. Assess means make some judgment about how great or small, beneficial or harmful etc. There are four marks so aim for at least four significant bits of information, and allow about 7 minutes for the question. There is probably 1 mark for naming the biopolymer, 1 for stating the impact(s) and 2 for making an assessment of the impact (along with reasons). The structure is not required but if you give it, it may help you get full marks if a later part of you answer is not quite up to scratch.
The only biopolymer treated in CCHSC that suits this question is poly(3-hydroxybutanoate). However you may have studied another one such as erthyropoietin

(p 27-9)

A recently developed biopolymer is poly(3-hydroxybutanoate), PHB which has the structure:

The impact of this development is that it has demonstrated that it is possible to use a biological process (biotechnology) to manufacture commercial quantities of a polymer, and that it is possible to make a biodegradable polymer and to make it from a non-petrochemical source (virtually all other synthetic polymers are non-biodegradable and come from petrochemicals, that is from oil).
The impact is quite significant, first because it has shown that biotechnology can be used to synthesise purely chemical substances, not just biological ones, and secondly because it has produced a simple biodegradable polymer. Most other biodegradable polymers are complex mixtures of synthetic and natural polymers. The ability to produce polymers from non-oil sources is also of considerable significance. However the impact of developing PHB has not been as great as had originally been expected in that the polymer is quite expensive and has found only limited uses.

Question 19 (4 marks)
Choose the cell you are most familiar with. The chemistry is of similar difficulty for both.	Cell X
	(a)	It creates a lot of solid waste and uses a lot of energy in its manufacture relative to the small amount of electricity it produces.
You could give a diagram such as Fig 2.6 on p 53 but it is not essential. The two half reactions are essential; the overall reaction is probably not necessary. The MnO₂ reaction could have been written as: MnO₂(s) + NH₄⁺(aq) + H₂O(l) + e^– ® Mn(OH)₃(s) + NH₃(aq) (p 53-4)	(b)	The cell consists of a zinc outer casing (the negative electrode) with a central graphite rod as the positive electrode. This rod is surrounded by a moist paste of MnO₂ and NH₄Cl separated from the zinc case by a paste of NH₄Cl alone. The zinc oxidises to Zn²⁺ releasing electrons to the external circuit: Zn(s) ® Zn²⁺(aq) + 2e^– These electrons flow through the external circuit and back to the carbon where they are absorbed by the reaction: MnO₂(s) + NH₄⁺(aq) + e^– ® MnO(OH)(s) + NH₃(aq) This leads to the overall reaction: Zn(s) + 2MnO₂(s) + 2NH₄⁺(aq) ® Zn²⁺(aq) + 2MnO(OH)(s) + 2NH₃(aq) As the cell is used the zinc casing dissolves and MnO₂ is consumed. This cell cannot be recharged.
	Cell Y
	(a)	Careless disposal of the lead accumulator (car battery) can result in the release of lead, a dangerous heavy metal, into the environment. It can also result in the release of sulfuric acid which will detrimentally lower the pH of soils and waterways.
You should point out that 'Cell Y' is actually a battery. A diagram such as Fig 2.12 on p 59 could be included but is not essential. Do not try to draw a diagram such as Fig 2.11(a): it is too complicated. The question is about the chemistry of the cell not the technology of designing a high current device. The two half reactions are essential, though the overall reaction is not necessary (p 58-9)	(b)	Cell Y is actually a battery of six 2 V cells in series to produce a 12 V battery. Each cell consists of two lead plates dipping into a solution of sulfuric acid, the electrolyte through which ions migrate. One lead plate is coated with PbO₂. This is the positive electrode. Both lead plates become covered with lead sulfate as the cell discharges. At the negative electrode Pb is oxidised to Pb²⁺ which is converted to PbSO₄ by the sulfuric acid: Pb(s) + HSO₄^–(aq) ® PbSO₄(s) + H⁺(aq) + 2e^–These electrons flow through the external circuit to the other plate, the positive electrode, where they are absorbed by the reaction: PbO₂(s) +HSO₄^–(aq) + 3H⁺(aq) + 2e^– ® PbSO₄(s) + 2H₂O(l) The overall reaction is Pb(s) + PbO₂(s) +2HSO₄^–(aq) + 2H⁺(aq) ® 2PbSO₄(s) + 2H₂O(l) As the cell operates both plates get covered with lead sulfate and the concentration of the sulfuric acid decreases. This cell can be recharged by passing electricity through it in the reverse direction.
Question 20 (7 marks)
A diagram is not required here, but the equation is. (p 33-4) A diagram is essential here. (CCPC p 13) Both the equation and diagram are essential here. (p 175-6)	Glucose ® mixture containing ethanol The chemistry of this step is the fermentation of glucose to ethanol using yeast. C₆H₁₂O₆(aq) ® 2CO₂(g) + 2C₂H₅OH(aq) Yeast and nutrients are added to an aqueous solution of glucose at about blood temperature in the absence of oxygen and fermentation occurs until about a 15% solution of ethanol is formed. ® pure ethanol Pure ethanol is obtained from this mixture by distillation. (Draw a diagram such as Fig 1.5 on p 13 of CCPC.) Distillation is the process in which a mixture is boiled to drive off the more volatile component which is condensed back to liquid which flows into a different container. ® ethyl butanoate Butanoic acid, ethanol and a small amount of concentrated sulfuric acid as catalyst are placed in a flask and heated under reflux for some time to form the ester ethyl butanoate: CH₃–CH₂–CH₂–COOH + C₂H₅OH ® CH₃–CH₂–CH₂–COO–C₂H₅ butanoic acid ethyl butanoate Heating is necessary to make this reaction go reasonably quickly. Refluxing is the process in which a mixture is heated with a cooling condenser attached so that any volatile component that vaporises gets condensed back to liquid and flows back into the mixture. (Draw a diagram such as Fig 5.7 on p 176.)
Question 21 (7 marks)
Analyse (identify components and the relationships among them; draw out and relate implications) is an odd choice of verbs here. Explain (relate cause and effect) would seem more appropriate – even discuss or outline would be better, but I guess the examiners feel that they must use as many as possible of the BOS list of verbs as they can – a hazard for HSC candidates! Basically what this question is wanting you to do is to show (explain, describe) how developments in our knowledge of the composition and properties of acids have lead to changes in our definitions of acids. The key definitions are those of Davy, Arrhenius and Bronstëd and Lowry. You could start with Lavoisier, though his definition was soon shown to be wrong. (p 146-8)	Originally acids were substances with a sour taste (vinegar, lemon juice). The first attempt to interpret acidic properties in terms of chemical composition was by Lavoisier who proposed that acids were substances that contained oxygen. As the composition of more and more substances became known, this was shown to be incorrect (hydrochloric acid did not contain O; calcium oxide, a base did). After observing that all acids contained hydrogen and that acids commonly reacted with metals to produce hydrogen gas, Humphry Davy defined acids as substances that contained replaceable hydrogen (that is they could react with other substances to form compounds (salts) that did not contain hydrogen). This definition worked well for many decades. After it became known that aqueous solutions of acids readily conduct electricity, Arrhenius proposed a definition that was based upon their properties, not on their composition: acids are substances that ionise in solution to produce hydrogen ions. However this definition did not adequately handle a further property that emerged, namely that the solvent that the substance was dissolved in helped determine whether or not the substance was an acid and whether it was strong or weak. To accommodate this property Bronstëd and Lowry introduced a new definition: acids are substances that tend to give up a proton to another substance such as the solvent (that is acids are proton donors). For example: HNO₃(aq) + H₂O(l) ® H₃O⁺(aq) + NO₃^–(aq) This is the currently preferred definition of an acid. It has emerged first from a knowledge of the composition of acids then from an understanding of their properties.

You need to recognise that for the first five points zinc is the limiting reagent (not enough of it to use up all the HCl) but that for the last three points Zn is in excess so using more Zn will not increase the volume of gas formed.	(b)	380 mL. The same volume of gas was formed from 1.22, 1.64 and 1.93 g zinc. This must mean that these three masses are more than enough to use up all the hydrochloric acid; that is, zinc must be in excess. So using even more zinc, such as 3.00 g, will still only produce this 380 mL gas.
The calculation refers to the region where Zn is the limiting reagent so the volume of gas depends upon the mass of Zn used. You need to show the equation to establish that 1 mole Zn produces 1 mol of gas. You would lose a mark if you did not show the equation even if your answer was correct. Show all your working for the calculation; there is space for it. And give your answer to the correct number of significant figures, two because there are two in 0.56. Hence it is better to leave your answer in litres rather than mL where you might be tempted to write 212 mL (even 210 is ambiguous). (p 128-31)	(c)	The graph shows us that 0.56 g zinc is not enough to use up all the HCl so zinc is the reactant that determines the amount og gas that is formed. The equation for the reaction: Zn(s) + 2HCl(aq) ® H₂(g) + ZnCl₂(aq) shows that 1 mole Zn ® 1 mole H₂ gas.
Question 23 (2 marks)
Other reasons are possible, such as poor mixing, so that insufficient oxygen gets to the fuel, a common problem with solid fuels such as coal or wood.	(a)	Lack of sufficient oxygen
This is the simplest equation that could be written. You could write it as CH₄(g) + 1½O₂(g) ® CO(g) + 2H₂O(g) Other equations are possible such as 2CH₄(g) + 3½O₂(g) ® CO₂(g) + CO(g) + 4H₂O(g) The important points are that water is the other product and that your equation is balanced. (p 199 and CCPC p 286-8)	(b)	2CH₄(g) + 3O₂(g) ® 2CO(g) + 4H₂O(g)
Question 24 ( 5 marks)
You could have written Ca²⁺(aq) + 2Cl^–(aq) instead of CaCl₂(aq).	(a)	CaCO₃(s) + 2HCl(aq) ® CaCl₂(aq) + CO₂(g) + H₂O(l)
Show all your working. Use the correct number of significant figures: there are three in 25.0, 0.600, 14.2 and 0.100 so three in your answers. (p 160-2. Back titration is explained in the HSC Module 3 page of this website; click here to go there)	(b) (c)
	(b) (c)
Question 25 ( 6 marks)
	(a)
Although chloride and sulfate generally produce precipitates with lead ions in solution, if the concentration of lead is quite low as is likely to be the case in environmental water, no precipitate may form with them The iodide test is much more sensitive. The question does not ask for an equation but with two marks for it, it may be worth including one particularly as it is such a simple one.	(b)	Add drops of colourless potassium iodide to a sample of the water and shake . If a yellow precipitate forms or if the mixture develops a yellowish turbidity (milkiness), then lead is present (as Pb²⁺). Pb²⁺(aq) + 2I^–(aq) ® PbI₂(s)
You could equally well discuss copper, zinc cadmium and lead in water and foodstuffs. Lead is probably not a good choice as it is involved in (b). Phosphate in waterways needs to be monitored because high concentrations can set off algal blooms which are very damaging to dams and streams. Calcium and magnesium in water need to be monitored because they cause hardness in water which can lead to scale buildup in factory pipes. (p 219)	(c)	Mercury ion, Hg²⁺, in fish needs to be monitored because mercury is poisonous to people. Mercury is widely dispersed throughout the oceans at very low concentrations that are of no great danger to people's health. However it can become concentrated up the food chain and so can reach dangerous concentrations in species at the top of the food chain such as tuna and shark. Hence we need to monitor its concentrations in fish to protect the health of people.
Question 26 (7 marks)
Describe the sources, not just name them, describe methods of purifying the water, and make sure that you assess those methods; that is state how effective they are. There are other sources of contamination in the diagram such as biochemical oxygen demand (BOD) from farm runoff, high concentrations of phosphate (from farm fertiliser runoff) and oil contamination from boating (though normally boating is not allowed on lakes and dams used for town water supplies.) Before choosing the two sources to describe, think ahead to the second part of the question, methods of purifying the water, and so select the sources you know how to purify. The purifications you describe should be for the contaminants you list; the two parts of the question are not independent. You could include a diagram such as Fig 8.4 on p 291 but it is not essential. (p 262-4, 289-91)	One source of contamination is suspended solids or turbidity which gives the water an unattractive appearance and an unpleasant taste. This would be caused as rain water flowed across the logged forest and washed loose soil into the river and lake. Much of this would stay suspended in the water as fine particles. A second source of contamination is water runoff from the farm after rain. This could carry various bacteria, viruses and parasites into the lake water. These are present in the faeces and urine of the farm animals and birds and some of them are harmful to people. To make the water from the lake suitable for domestic use in the town at least two types of water treatment are needed, one to remove suspended solids (turbidity) and the other to remove or destroy the disease-causing agents. To remove turbidity an Al³⁺ or Fe³⁺ salt is added to the water and the pH is adjusted if necessary in order to produce a precipitate of Al(OH)₃ or Fe(OH)₃. As this extremely insoluble precipitate forms as very small particles and coagulates into larger ones, it adsorbs suspended solids from the water and so those solids precipitate out with the hydroxide. The precipitate is then filtered off, generally through a coarse sand filter and clear water results. This process is very effective. To remove bacteria and viruses and some parasites the clarified water is treated with chlorine which destroys bacteria and many viruses and is an effective way of sanitising the water and making it safe to drink. While chlorination is an effective method of sanitising water, it is not foolproof. Some viruses and many parasites (such as Giardia and Cryptosporidium) are not destroyed by the low doses of chlorine that are used to treat domestic water supplies. Hence water authorities try to eliminate these dangers by keeping human activities out of the water catchment area and by carefully monitoring the water for such substances.
Question 27 ( 5 marks)
	(a)	The reaction has reached equilibrium
You should show the vertical jumps as the volume is suddenly decreased at time T₂, then show how the three concentrations change as equilibrium is re-established. It probably was not necessary to get the relative changes quantitatively accurate, just so long as they were qualitatively in the right directions, H₂ and N₂ decreasing and NH₃ increasing and them all coming back to equilibrium. I chose a 50% increase in concentration so my NH₃ and N₂ curves would not get tangled up. The explanation really requires a chemical equation and mention of Le Chatelier's principle. (p 117-9, 200-1, 315-6)	(b)	(i) (ii) As soon as the volume was reduced, the total pressure increased and so all the concentrations (partial pressures) increased by the same proportion; for example a 50% increase in all the concentrations. These are the vertical increases shown in the graph at T₂. These changes in pressures cause the equilibrium, N₂(g) + 3H₂(g) 2NH₃(g) to adjust. Le Chatelier's principle tells us that the reaction moves in the direction that opposes the change, that is in the direction that reduces the total pressure which is from left to right, because in that direction 4 moles ® 2 moles. Hence the concentrations of N₂ and H₂ decrease and that of NH₃ increases. This is shown in the graph. The decrease for H₂ is three times that for N₂ and the increase for NH₃ is twice the decrease for N₂ because of the molar ratios in the equation.

Question 16 (6 marks)
Name the compound you actually used. Do not say ethene, propene or butene; these are all gases and so you would not have used them in the lab.	(a)	1-hexene (or possibly 2- or 3-hexene or 1- or 2- pentene or cyclohexene)
1 mark for the hazard, 1 for the safety precaution	(b)	Flammability of the alkene. We made sure that there were no naked flames or hot plates in the lab. (Alternatively, high concentrations of alkenes can be detrimental to people's health. Hence we were careful not to inhale the liquid or to spill it on our hands. If possible perform the experiment in a fume hood.)
The description of this simple experiment does not seem to be much for two marks, but that is specifically what you were asked to do so there is no point in giving extra information. Surprisingly you were not asked for an equation so one was not needed. (p 9-10)	(c)	We put a few mL of the liquid to be tested into a test tube and added drops of a brown solution of bromine in hexachloroethane to it with shaking; our alkene decolorised the solution. (Alternatively you may have used an aqueous solution of bromine in which case the mixture in the test tube would have gone slightly cloudy, though all the brown colour would have been discharged.)

Question 17 (3 marks)
	(a)	Some of the heat released from the burning of the ethanol was lost to the surroundings instead of heating the water. (Alternatively: the combustion of the ethanol did not go completely to CO₂; some ended up as CO and so less heat was released than should have been.)
How many significant figures should you give in your answer here? That depends in how many figures there are in the 200 mL. Is it 2.00 x 10², 2.0 x 10² or 2 x 10²? So 1, 2 or 3 significant figures have to be accepted here (but not 4 or more!) (p 36-7)	(b)

Question 22 (7 marks)
Plot the graph the correct way around. The volume of gas depends upon the mass of zinc, so mass is the independent variable (x axis) and volume is the dependent one (y axis). Choose suitable scales to use as much of the grid as possible. Label the axes with their names and scales. Plot points finely but identify them with circles or squares etc. Draw two straight lines through your points. Ignore the outlier, obviously a mistake.	(a)